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Actually, it gave me some clues, but didn't answer all my interrogations. Per exemple, the ports is de facto tuned to Fb. I can't find how to tune the port to the driver's Fs 20Hz, or somewhere between Fs and Fb.

Changing the size (typically the length) of the port-tube you can change "tuning of the WHOLE SYSTEM", (box+port resonant frequency Fb) and what would be the influence of such (tuning or changing) to the WHOLE SYSTEM RESPONSE, you can easily see using mentioned simulation programs.
Here are some examples:
it seems that "what-ever-you-do" you will get -6dB around 30Hz....
so it seems to me that optimum in maximal cone displacement "in band", and "out of band" would be the 'leading rule'.

Changing the size (typically the length) of the port-tube you can change "tuning of the WHOLE SYSTEM", (box+port resonant frequency Fb) and what would be the influence of such (tuning or changing) to the WHOLE SYSTEM RESPONSE, you can easily see using mentioned simulation programs.

Yes. That I know. Only I haven't yet been able to find how to do it with the Excel simulator I have.

Once You are used to the table in the Thiele paper it will take You less than a minute to understand what is possible with a certain drver, and what not.

I personally use Bass Box Pro these days.

The tables were all we had back in the 70's and 80's. While simulation software has made using them all but obsolete I do think it's important to know that those tables are the basis for the simulation software.

An interesting side bar, one rather famous JBL engineer was an expert in theory but couldn't design a good sounding loudspeaker if his life depended on it. Another JBL engineer wasn't all that high on theory but he sure could design some great sounding systems. Both still work for Harman after all these years.

Subwoofer port tuning

Originally Posted by ivica

Changing the size (typically the length) of the port-tube you can change "tuning of the WHOLE SYSTEM", (box+port resonant frequency Fb) and what would be the influence of such (tuning or changing) to the WHOLE SYSTEM RESPONSE, you can easily see using mentioned simulation programs.

This puts the system frequency (fm) at about 26 Hz which is as per the article.

Yet the fact that the impedance peaks of port and speaker do not match up as they should in a proper tuned system got me confused initially. Therefore I reduced the port length stepwise to get them aligned. It turned out the system requires a port length of about 9 cm + baffle (4"), resulting in a system frequency of 33.5 Hz, again as per the article.

Re-reading the article I now understand that the BX63 filter, electronically compensates for the "error" of the longer port. Though I do not exactly grasp how the bass boost makes the impedance curves of port and speaker match.

...............
Re-reading the article I now understand that the BX63 filter, electronically compensates for the "error" of the longer port. Though I do not exactly grasp how the bass boost makes the impedance curves of port and speaker match.

BX63 compensate (boost) lower frequency region taking care of mechanical driver limitations ( Xmax). It does not have any influences of the diver(in the box) impedance.

I 've been calculating a 4th order bass reflex cabinet at 10,1cft using Vb=(15)(Vas)(Qt2.87)

Then found F3 32,5Hz using F3=0.26(Fs)/(Qt1.4)

And Fb 27,3Hz (basically the port tuning) with Fb = 0.42(Fs)/Qt 0.9

What if I increased cabinet size to 12cft?
How lower will F3 be? Any drawback?

I want to build two cabinets to use up to 800Hz for home duty. No PA action here.

<What if I increased cabinet size to 12cft?

Asbolutely no advantage going from 10 to 12 cu ft with an un assisted alignment in a normal domestic situation.

>Any drawback?
Yes. Size of enclosure and what is known as group delay in the base response. JBL do not recommend this size enclosure operater above 80 hertz

Actually, it gave me some clues, but didn't answer all my interrogations. Per exemple, the ports is de facto tuned to Fb. I can't find how to tune the port to the driver's Fs 20Hz, or somewhere between Fs and Fb.

So far it goes like that. 10cft vs 12 cft tuned at Fb 27,3Hz

Response. The thin green line is the gain/loss between 10 and 12 cft

Your simuation is incomplete.

You need to know where the enclosures will be located and thw resulting effect of room gain in your room.

The effect of room gain is going to have a much greater impact on the overall reponse than incremental tuning differences.

When this is accounted for add the room gain to the driver/enclosure simulated response.

To give you a few clues if you are considering using the 2245 above 80 hertz you will want the smoothest bass in the pass bandband response from 35 - 300 hertz.

The effect of room gain is to create a hump that decends from 120 hertz to 40 hertz with the 2245 in 8 cu ft tuned to 28-29 hertz ie maximally flat response.

According to Witold Waldman a scientist in Aust who created Calsod the 1st system optimisor software written in DOS, the human ear is particularly sensitive to distortion and response flatness below 100 hertz.

If your amp has a bass tone control this is quite obvious.

The size of the hump will vary according to the enclosure location.

You can triangulate the Q and magnitude of the hump according the height of the woofer centre above the floor and distance from the wall boundary.

The 2245 has strong boundary reinforcement so it is desirable to have the woofer centre at least 15 inches above the floor level

Given the size of this enclosure you are not going to have too many placement options so we will assume its located against a floor / wall boundary. Corner placement will exagerate the size of the bass hump.

In other words it will sound more woolly and possibly boom.

The idea is to use the room gain to your advantage to extend the system response with minimal ripple or bass hump.

So how do we fix this without applying active equalization?

By moving to a larger box and a lower tuning frequency we can see the tilt in the response below 100 hertz which is advantagous to compensate for room gain.

To tune the system correctly you cannot however pick any combination of the enclosure size and FB and we refer the the formula below.

Sub optimal tuning will response in ripple in the passband. The effect of this is poor transient performance.

We also need to consider the dymanic performance and the displacement demands on the woofer in the 35-50 hertz range where the port is going to actively contribute the systems overall dynamic range.

If you tune too low you will loose some of the benefit of the port in the useful range of 35-50 hertz.

The un assisted tuning below is a very good balance of box size, bass extension in room and in room response flatness.

I have used this alignment with the 2245 and in practise ands its better for the above reasons than the 4345 box tuning.

The beauty of a diy project is you are free of marketing and manufacturing contraints and can design the system to suit you room and your needs.

Enjoy and have fun.

10,1cft using Vb=(15)(Vas)(Qt2.87)

F3 32,5Hz using F3=0.26(Fs)/(Qt1.4)

Fb 27,3Hz (basically the port tuning) with Fb = 0.42(Fs)/Qt 0.9

Remember the Vb is the net volume allowing for bracing, displacement volume of the woofer and effect fill which increases the virutal enclosure Volume due to frictional losses QL.

In an enclosure of this size the effect of normal fill ie 1 inch fibre glass will cancel out the volume subracted for the bracing and the woofer.